I.C.P. is specifying a CofG 30–38 % m.a.c. for their Savannah 'VG'....
Is this a realistic CofG range for such a wing???
History and experience with most wing profiles including the well-known NACA 650-18 indicates 20-30% to be the usual range. The Savannah VG wing is not a true 650-18 (see 'Wing Comparisons' page), but close enough that the CofG range is likely to be unchanged. I.C.P. claim that they chose this airfoil because of published data on it, but that data indicates a CofG range of 20-30%......
I’ve been hearing of new Savannah VG builders having problems attaining the 30-38% range specified by the manufacturer, and moving the battery to the tail to try to reach that range.
Is this a correct move???
My Savannah, with the original wing with VGs instead of slats, weighs in at 22 % forward and 28 % aft, and flies perfectly balanced, even with the prop stopped and airspeed below 30 kts. This is with 400 hrs of flying experience in this configuration, much of it intensive flight testing at the edges of the flight envelope while testing VGs. If I was to add the new leading edge supplied by I.C.P. that extends the leading edge approx. 66mm forward, that would make my loaded CofG to be 31 % m.a.c. Then if I were to put the battery in the tail, that would become more like 34 % m.a.c. ...... This would be in the range specified by I.C.P., but all my own flying experience, and published aerodynamic history for other aircraft, indicates that I sure wouldn’t want my CofG to become 6% farther aft than it is right now......
A high-wing aircraft like the Savannah, is very tolerant of loading outside the range - so long as it’s flying at speed and the aerodynamic forces are dominant, but engine stopped and get real slow and then gravity and balance become the dominant influences. With an engine-out and no prop wash over the elevator, an aft CofG at full load can get into a tail-heavy stall when you can’t get the nose down to maintain airspeed....... that can end in a very heavy pancake ‘splat’ or worse yet a flat spin........
The Savannah aircraft itself, as assembled from that kit with all the precisely-made parts, and a 912 series engine, with the battery right behind the seat, is well-balanced.
It's only the recommended CofG range that's in question???????
Seems to me we need I.C.P. to explain how they justify such an unusual recommended range for this well-established wing..........
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Weight and Balance for my Savannah Aircraft
Empty (Weighed) 290 kg @ 17 % m.a.c.
Minimum Load (Calc.) 363 @ 22 %
Full Load (Calc.) 540 @ 28 %
Travel Load (Calc.) 468 @ 27 %
Travel Load (Weighed) 473 @ 28 %
Notes:
- This is with the Original wing profile, without slats, m.a.c. = 1270 mm.
- Datum – front edge of leading edge of wing.
- Empty (Weighed) was as constructed, before first flight, battery behind the seat.
- Minimum Load (Calc.) is with 73 kg pilot + 10 kg (15 litres) fuel.
- Full Load (Calc.) is with 90 kg pilot + 90 kg pax + 50 kg fuel + 20 kg baggage.
- Travel Load (Calc.) is with 73 kg pilot + 35 kg fuel on pax seat + 50 kg main fuel + 20 kg baggage.
- Travel Load (Weighed) was actually weighed on professional quality scales with the indicated loads in place. 5 kg more than calculated due misc weight gain.....
I'm really fussy about weight and balance and CofG. I don’t know how many other aircraft owners have been this thorough in weighing, calculating W&B, and re-weighing their aircraft, but I sure haven’t found any. Most just went through the initial weigh-in, often with confusing results that don’t really match the chart provided, many without really understanding the process at all, and when I ask what their present weight and balance is, they just say, “I don’t know, but it flies OK.....”. Not good enough I reckon........
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